A simple laptop for your son, sounds a fair swap - can he do the washing up & the ironing?
Have a look at the Dell ones, ISTR they are doing good deals.
For what your saying, you really only need the basic, although I would look at
ones that have 2gb of RAM instead of 1gb, certainly ignore any that are only on 512mb. Especially if it comes with Vista.
Hard drive size, 80gb should suffice & if you fill that, external drives are relatively cheap.
Most internet games will play on most modern pc's however some, esp the likes of world of warcraft etc can really take a toll on the PC. SO can you be more specific about what games he will play?
Just satisfying the minimum requirements for them often means that they run dog slow with very low graphics.
For his homework your also likely to want some software, so what does he use at school? MS Office 2007 by chance? you'll need that & it can be costly even for a student license. (although there are other sources for it).
You want to be able to play poker on it? well I certainly hope that you enable multi user logons and ensure that for your account your son cannot get access to it as he may be able to make some bets using your money if he can. Give him his own login & remove all access to any betting programs, you may have to move things out of the "all users" folders to do that.
Dont want to sound patronising but if your playing poker, your a gambler? in that case dont spend too much on the laptop as you'll be needing it later